On applying hot dip galvanization to a strip-shaped steel sheet, the steel sheet which was treated by pickling for descaling, followed by rolling in a rolling mill to a specified thickness is annealed in an annealing furnace, and further is transferred to a molten zinc bath. FIG. 3 illustrates the process of common hot dip galvanizing line on and after the molten zinc bath. The steel sheet travels in the arrow “a” direction.
For applying hot dip galvanization to a steel sheet 1a, the steel sheet 1a is immersed in a molten zinc bath 2, as illustrated in FIG. 3. Zinc in molten state is held in the molten zinc bath 2, (hereinafter referred to as “the zinc bath”). During the travel of the steel sheet 1a in the zinc bath, zinc adheres to both surfaces of the steel sheet 1a. 
Then, the steel sheet 1a is sent from the molten zinc bath 2 to an alloying furnace 3, where the steel sheet 1a is subjected to alloying treatment. The alloying treatment is a heat treatment to enhance the alloying reaction between the steel base material of the steel sheet 1a and the zinc adhered to the steel sheet 1a, thus forming a zinc-plating layer having excellent adhesion.
The steel sheet 1a after leaving the alloying furnace 3 is cooled before an interim looper 4 while being adjusted in the tension thereon, and is further sent to a temper rolling mill 5 to undergo temper rolling (what is called the “skin pass”). The temper rolling is a rolling to apply a light reduction of about 0.6 to about 3% of reduction in thickness to deform only in the vicinity of the surface of steel sheet 1a, thereby adjusting the surface properties (such as surface roughness) of the steel sheet 1a. The reduction in thickness is defined by the value derived from eq. (1):Reduction in thickness (%)=100×(t1 −t2) /t1  (1)where, t1 is the thickness before temper rolling (mm), and t2 is the thickness after temper rolling (mm).
Then, the steel sheet 1a is fed from the temper rolling mill 5 to a surface oxidation apparatus 6 to receive surface oxidation treatment. The surface oxidation treatment is given to bring both surfaces of the steel sheet 1a into contact with an acidic solution, thus to form an oxide film on the surface of the plating layer. The steel sheet which was treated by the surface oxidation is hereinafter referred to as “the hot dip galvanized steel sheet 1b”. 
With thus covering the plating layer with the oxide film, the sliding property of the hot dip galvanized steel sheet 1b on working (for example on press-forming) into products having varieties of shapes is improved. Since, however, the hot dip galvanized steel sheet 1b which is processed from the surface oxidation apparatus 6 has acidic solution adhered thereto, both surfaces of the hot dip galvanized steel sheet 1b are cleaned in a rinse tank 7 by washing off the acidic solution, and the hot dip galvanized steel sheet 1b is further dried in a drier 8.
The cleaned hot dip galvanized steel sheet 1b enters an outlet looper 9, where the tension thereon is adjusted, and enters an oiler 10, where a rust-preventive is applied onto the surfaces thereof, followed by coiling the hot dip galvanized steel sheet 1b by a coiler 11.
As of the conventional hot dip galvanizing line described above, FIG. 4 shows a part-enlarged view ranging from the surface oxidation apparatus 6 to the rinse tank 7. The hot dip galvanized steel sheet travels in the arrow “a” direction.
The surface oxidation apparatus 6 brings the surface of the hot dip galvanizing on the steel sheet 1a into contact with the acidic solution. As shown in FIG. 4, for example, acidic solution spray nozzles 12 to spray the acidic solution 13 are arranged therein.
The hot dip galvanized steel sheet 1b on which the acidic solution was sprayed in the surface oxidation apparatus 6 is sent to the rinse tank 7. To assure a period of time necessary to form the oxide film on the surface of the plating layer at a sufficient thickness, the distance between the surface oxidation apparatus 6 and the rinse tank 7 is determined to a specific length. For example, by controlling the traveling period of time between the surface oxidation apparatus 6 and the rinse tank 7, the thickness of the oxide film can reach to 10 μm (nanometer) or larger. Japanese Patent Laid-Open Nos. 2002-256448 and 2003-306781 disclose that the covering a plating layer with an oxide film having 10 μm or larger thickness improves the sliding property of hot dip galvanized steel sheet 1b, thus preventing damages and peeling of plating layer on working (press-forming and the like) into products having varieties of shapes.
The rinse tank 7 arranges nozzles therein to spray a cleaning water 14. By spraying the cleaning water 14 to the hot dip galvanized steel sheet 1b, the acidic solution adhered to the hot dip galvanized steel sheet 1b is removed. Sole spraying of the cleaning water 14 is, however, difficult to completely wash off the acidic solution adhered to the hot dip galvanized steel sheet 1b. Although investigations about the issue are given including addition of chemicals to the cleaning water 14, there are left improvement issues in terms of composition and adding amount of chemicals.
Remained acidic solution on the surface of the hot dip galvanized steel sheet 1b leads to corrosion of the plating layer by acid, which results in not only the deterioration of appearance but also the damages and peeling of plating layer, thereby decreasing the product yield.
An object of the present invention is to solve the above problems and to provide a cleaning method and a cleaning apparatus to efficiently and fully wash off the acidic solution adhered to the surface of a hot dip galvanized steel sheet which was treated by surface oxidation.